The present invention relates to a cleaning device for use in an image forming apparatus.
In general, in an image forming apparatus such as during the completion phase of a copying cycle, any electronic copying apparatus, an untransferred toner (residual developer) remaining on the surface of a photosensitive drum is scraped off by means of a cleaning blade (cleaning member).
If a great frictional force is produced between the photosensitive drum and the cleaning blade, the drum surface may be damaged or filmed with the toner, or the blade may be burred due to stress, failing to keep in contact with the drum. Moreover, an electrostatic effect may be caused by the friction and impair a uniform surface potential during a charging process, resulting in uneven or irregular images or fog. The frictional force can be reduced by lowering the cleaning blade pressure. If this is done, however, the cleaning blade cannot fulfill its primary function, i.e., cleaning the photosensitive drum, thereby causing filming.
Conventionally, therefore, lubricant has been added, by way of trial, to the developer for the purpose of reducing the frictional force between blade and drum. However, the toner used therefor may be lowered in fixing capability by the lubricant therein, or the photosensitive drum may be coated with a thin toner film. Thus, the image quality is subject to substantial variations and mades this suggested arrangement very impractical.
Also, other lubricants have hitherto been tried as a coating material to be applied to the surface of the photosensitivie drum. There has also not been any method of restricting a lubricant film on the drum to a thickness such that the electrostatic characteristic of the drum not is lowered, that is, a thickness of hundreds of angstroms or less. Moreover, there have been attempts to improve the material of the blade by using one with a low coefficient of friction. However additives impair other desirable characteristics of the blade, especially its mechanical strength.
In the prior art initially setting the photosensitive drum usually produces great stress between the blade and drum, consequently subjecting the blade to burr and damaging the drum. To avoid this, an operator or serviceman sprays lubricant powder, such as polyvinylidene fluoride, to a thickness of 5 micrometers on the drum, thereby easing the friction between drum and blade. This manual work is troublesome and lacks in reliability. For example, the operator may touch the drum during the work.
Alternatively, a toner layer may be formed on the photosensitive drum at the time of the initial drum setting. According to this system, the drum is first rotated, and at the same time, a toner layer is formed on the drum during period A with the developing bias as an inverse bias, and the blade is then brought into contact with the drum during period B with starts during the second revolution of the drum, as shown in FIG. 1. Thus, the toner layer is utilized as a lubricant. In this case, however, a high frictional resistance is still produced on that portion of the contact region between drum and blade with no toner layer, causing burr of the blade or damaging the photosensitive drum.